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I put Lunar month on the list because there's confusion in general about how pregnancy months "work", and sometimes people try to explain their bad math by saying 280 days of pregnancy is "10 lunar months". This is not true, because a lunar month is not 28 days. A calendar month is also not 28 days. There are not special "pregnancy" months that get to be 28 days. Nothing about anything with the word "month" in it is 28 days.
This means that there's no month that equals four weeks, so you can't find out how many months you are by dividing by 4. Don't ask me why this is such a big deal to me, I just hate it when people get their weeks and months wrong. Maybe it's a pregnant engineer thing?
So let's talk about what's right...
The most correct thing would be to talk about your months in terms of calendar months. If your last period was September 4th, then on October 4th you're one month pregnant, November 4th you're two months, December 4th you're three months, etc.
The "mathy" thing to do would be just use assume the average month is 1/12th of a year, or 30.4 days. A week is 7 days. So there are 30.4/7 = 4.34 weeks in a month.
Either method means that the weeks don't nicely evenly fall when the months do... and that's okay! For example, you're now at 21 weeks, which is one of those weird ones. At the beginning of being 21 weeks pregnant, you're 4 months pregnant. At the end of being 21 weeks pregnant, you're 5 months pregnant.
I've heard people say they were 4 months pregnant because they were 16 weeks along... nope. I've had them tell me they were 5 months pregnant when they were only in their 5th month... there's definitely a difference there. During your first year of life, you're not 1 year old, right?
People we're bringing new life into the world. The least we can do is check our math!